In recent years a technology has been sought for realizing optical integrated circuits in which optical components are integrated, similar to integrated circuits in which electronic components are integrated. Optical circuits are currently assembled by connecting optical components such as optical switches, wavelength filters, and 3-dB couplers (optical couplers) by means of an optical waveguide such as optical fiber. If a plurality of optical components are integrated in a small chip, the volume, power consumption, and fabrication costs of the optical circuit are dramatically reduced.
Various types of techniques aimed at realizing optical integrated circuits have been proposed, one being a technique that uses photonic crystal. In the present specification, the term “photonic crystal” is used as a general term for a structure having a periodic refractive index distribution.
A photonic crystal has various special optical characteristics arising from the periodic structure of refractive index distribution. The most representative characteristic is the existence of a photonic band gap (PBG).
Normally, light is propagated through a crystal. In a photonic crystal having a periodic refractive index distribution, however, the phenomenon occurs that light which belongs to a particular specific frequency band cannot be propagated. Frequency band (or wavelength band) of light that can be propagated in a photonic crystal is called the photonic band. A plurality of photonic bands normally exists. The frequency of light that cannot be propagated in a photonic crystal are called photonic bands gap (PBGs) since they are the gaps existing between photonic bands.
When a defect that slightly breaks the periodicity of a refractive index distribution is present in a photonic crystal having a PBG, light of the frequency band of the PBG is trapped in the vicinity of this defect. As a result, in a photonic crystal in which a defect is arranged in a line, light of the PBG frequency band is guided along the defect that is arranged in a line. Accordingly, this photonic crystal can be used as an optical waveguide.
In the present specification, being part of the dielectric that is arranged in a lattice form in a photonic crystal, the dielectric that is arranged in a line and that corresponds to a lattice defect that disrupts the periodic structure of the refractive index is called a line-defect. In addition, being part of the dielectric arranged in a lattice form, the dielectric other than line defects is called non-line defect. An optical waveguide composed of the above-described photonic crystal is called a line-defect waveguide.
The guiding state of light in a photonic crystal is specified by the set (f, k) of frequency f and wave number k. Normally, light having electromagnetic field distribution that has identical symmetry or identical characteristics forms a band composed of continuous frequency and continuous wave number. This continuous band is called a “mode.”
The mode of guided light that is guided along a line defect while being confined to the vicinity of a line defect is called the “guiding mode (or waveguide mode).” The mode of light that is propagated in a medium that is separate from a line defect has similar characteristics to the mode of light that is propagated in a photonic crystal lacking line defects and is called “bulk mode.”
To use photonic crystal as an optical waveguide that guides light in three mutually perpendicular directions, a photonic crystal that has a structure in which the refractive index distribution is periodic in three dimensions must be used. However, a photonic crystal of a three-dimensional periodic structure has a complex structure and is extremely expensive to fabricate.
A photonic crystal having a structure in which the refractive index distribution is periodic in two dimensions (hereinbelow referred to as “two-dimensional photonic crystal”) is often used. The two-dimensional photonic crystal has a PBG with respect to light that propagates in a two-dimensional plane in which the refractive index is periodic. Accordingly, the presence of a line defect arranged in a line in this two-dimensional plane results in an optical waveguide in which light is guided along the line defect.
In a two-dimensional photonic crystal, the refractive index is not periodic in the direction perpendicular to the two-dimensional plane in which the refractive index is periodic (hereinbelow referred to as the “direction of thickness”). In this case, the total reflection caused by the difference in refractive index between the medium that makes up the photonic crystal and the surrounding medium is used to confine light that propagates in the direction of thickness.
An optical waveguide that uses the above-described two-dimensional photonic crystal is disclosed in Non-Patent Document 1 and Patent Document 1.
The optical waveguide disclosed in Non-Patent Document 1 is composed of photonic crystal in which rod-shaped dielectrics are arranged in a square lattice. This photonic crystal has 13 rows of dielectric rods. The diameter of the dielectric (line defect) of the center row (seventh from the end) is smaller than the diameter of the dielectric of the other rows.
In the following explanation, the dielectric rods that correspond to the line defect are referred to as “line-defect rods.” In addition, the dielectric rods other than the line-defect rods are called “non-line-defect rods.”
As with an optical fiber that functions as an optical waveguide because it has core and cladding, an optical waveguide composed of photonic crystal functions as an optical waveguide due to the existence of line-defect rods and the non-line-defect rods provided around the line-defect rods (and dielectric medium).
In an optical waveguide made up from a photonic crystal, it is generally believed that as many rows of non-line-defect rods as possible should be arranged on both sides of the line-defect rods. This is because such a configuration ensures a margin in the degree of attenuation to a negligible level of the electromagnetic field distribution of light that is confined in the vicinity of line-defect rods. Leakage of light from the optical waveguide can thus be prevented.
The schematic view shown in Non-Patent Document 1 shows non-line-defect defect rods arranged in two rows on each of the two sides of the line-defect rods that are arranged in one row. Only a few rows of non-line-defect rods are shown for the sake of simplifying the figure. In a microscope photograph shown in the non-patent document, the rows of non-line-defect rods are arranged in six rows on each of the two sides of the row of line-defect rods. Thus, in optical waveguides of the background art that include Non-Patent Document 1, at least six rows of non-line-defect rods are arranged on each of the two sides of the line-defect rods.
The optical waveguides described in Non-Patent Document 1 and Patent Document 1 have somewhat imperfect structures. These imperfect structures indicate non-uniformity of the refractive index of the dielectric medium that makes up the optical waveguide, non-uniformity in the height and profile shape of the line-defect rods and non-line-defect rods, or roughness of the interfaces of the line-defect rods and non-line-defect defect rods.
In an ideal optical waveguide that does not have the above-described imperfect structure, waveguide mode and bulk mode are independent modes to each other. If imperfect structure exists, however, the imperfect structure causes light that is guided along line-defect rods to scatter, whereby the waveguide mode and bulk mode are coupled. As a result, light that is guided as the waveguide mode leaks to the bulk mode.
Light that leaks to the bulk mode may again leak to the guiding mode at other locations of the optical waveguide. In this case, light that has propagated through a plurality of different paths causes interference, whereby the transmission characteristic of the optical waveguide is decreased and noise increases.
Accordingly, guidable light is only light that belongs to the guiding mode that does not intersect with the bulk mode (in dispersion relation), and the problem therefore arises that the frequency band of guidable light is narrowed. It is therefore desired that the frequency band of guidable light be broadened.
Non-Patent Document 1: S. Assefa et. al, “Guiding 1.5 μm light in photonic crystals based on dielectric rods,” Applied Physics Letters, Vol. 85, No. 25, pp. 6110-6112, December 2004.
Patent Document 1: JP2005-091925A